FM Modulator phase-locked loop with FM calibration

Abstract

Apparatus to provide calibrated frequency modulation (FM) of frequencies generated in a phase-locked loop is disclosed. A phase-locked loop (PLL) includes a voltage-controlled oscillator (VCO) which is locked to a loop frequence signal frequency. A modulo-10 divider is included which adds integral multiples of two pi radians of phase to the output signal of the VCO whenever the modulus of the divider is reduced by an integer. This forces the PLL to step change the loop frequency by an integral multiple of the reference frequency. A sampling circuit senses and measures the change in the VCO control voltage resulting from the step change in the loop frequency. The change in the control voltage is a measure of the VCO gain at that frequency. The change in the control voltage is converted to a digital number which is loaded into a multiplying digital-to-analog converter (DAC) to adjust the gain of the DAC. The effective combined gain of the DAC and the VCO is constant and independent of the VCO gain over the entire range of the VCO thereby providing an equal amount of modulation from an FM signal at any frequency of the VCO.

We claim: 
     
       1. An electronic apparatus for calibrated frequency modulation of a multiplicity of frequencies which are desired multiples of a reference frequency generated in a phase-locked loop, said apparatus comprising: frequency generating means for producing an output signal with a frequency determined by the level of a control input signal;   counting means coupled to the output of the frequency generating means for producing an output signal having a frequency equal to the frequency of the signal at the input of the counting means divided by N;   phase comparison means coupled to the counting means for comparing the phases of the output signal from the counting means and the reference signal, and generating an error signal indicative of a phase difference therebetween, said error signal serving as the control input signal to the frequency generating means to thereby control the frequency of the output signal from the frequency generating means;   input summing means coupled to the output of the phase comparison means and to the input of the frequency generating means for summing a frequency modulation signal with the control input signal for producing frequency modulation of the output signal from the frequency generating means;   frequency change means coupled to the counting means, responsive to a frequency step change signal, for changing the frequency of the input signal to the counting means thereby forcing the phase-locked loop to step change the frequency of the frequency generating means output signal by a multiple of the reference signal frequency;   sampling means coupled to the input of the frequency generating means for detecting a change in the control input signal and providing an output signal linearly proportional to said change in the control input signal; and   amplitude adjustment means coupled to the sampling means and responsive to the output of the sampling means for adjusting the amplitude of the control input signal, said adjustment of the amplitude of the control input signal being proportional to the output of the sampling means.   
     
     
       2. Apparatus as in claim 1 wherein said frequency change means comprises a phase shift means interposed between the frequency generating means and the counting means, for shifting the phase of the input signal to the counting means by a multiple of two pi radians of phase thereby forcing the phase-locked loop to step change the frequency of the frequency generating means output signal by said multiple of the reference signal. 
     
     
       3. Apparatus as in claim 2 wherein said phase shift means comprise cycle addition means for adding at least one cycle to the input of the counting means thereby adding a multiple of two pi radians of phase to the input signal to the counting means. 
     
     
       4. Apparatus as in claim 3 wherein said cycle addition means comprise a divide-by-two flip-flop coupled to a three-modulus prescaler. 
     
     
       5. Apparatus as in claim 1 wherein said sampling means comprises a capacitor coupled to ground through an electrode switch, said capacitor detecting said change in the control input signal level. 
     
     
       6. Apparatus as in claim 5 wherein said sampling means further comprises an analog-to-digital converter coupled to said capacitor and to the amplitude adjustment means, said analog-to-digital converter converting said change in the control input signal level to a digital signal indicative of said change in the control input signal level. 
     
     
       7. Apparatus as in claim 6 wherein said amplitude adjustment means comprises a variable-gain amplifier responsive to said digital signal for amplifying the amplitude of the control input signal proportionally to said digital signal. 
     
     
       8. Apparatus as in claim 7 wherein said variable-gain amplifier comprises a multiplying digital-to-analog converter interconnected between said input summing means and said frequency generating means, said digital signal adjusting the gain of said multiplying digital-to-analog converter. 
     
     
       9. Apparatus as in claim 8 wherein said amplitude adjustment means further includes a voltage offset circuit coupled to the output of the multiplying digital-to-analog converter for providing an offset voltage, said offset voltage being summed with the output of the digital-to-analog converter, the level of said offset voltage being determined by the frequency of the frequency generating means output signal.